If Barack Obama wins the Democratic presidential nomination, the flap over the picture in which he is dressed as a Somali elder is very likely to be just the first of a number of disputes about what constitutes a legitimate tactic in this new political era.

In fact, it would be surprising if the whole question of what should and should not be out of bounds in the campaign does not become a more frequent feature in the general election fight against Republican John McCain.

Not because McCain has a history of using campaign tactics that might offend the typical voter; in fact, his reputation is that he is cleaner than the average politician.

Yet, given Obama’s momentum and lofty standing with the American people, any candidate trying to defeat him will have to suggest to voters that maybe the Illinois senator is not who they want in the Oval Office.

Obama has run as a new-era politician, one whose mixed-race heritage (African father, white mother from Kansas) is symbolic of his comfort with cultures outside the U.S. He says that background has prepared him to open a new age of relations with many nations upset at President Bush’s policies.

But with Obama as the first black candidate with a serious chance of winning the White House if he is nominated, the upcoming campaign will force all concerned to participate in a running conversation about what is fair and what is foul.

The photo of Obama in the traditional African dress surfaced on the Drudge Report, which said it had been supplied by Clinton aides. Obama’s campaign called it “fear mongering.”

Regardless of how the picture came into the public domain — although if the Clinton people did it and then lied about doing so, that would be a no-no — the more important question is whether making the picture available is out of bounds.

If Obama says his heritage makes him uniquely qualified to work well with leaders of non-Western nations, is it fair game for an opponent to show him in a picture that reminds American voters of his African roots and ties?

At least part of Obama’s recent success has been this appeal to core Democrats that he can reach out to those in the world alienated by Bush’s presidency. Democrats, especially primary voters, are more accepting of different cultures than the nation as a whole.

But there are surely tens of millions of American voters who are much less comfortable with Obama’s background. The photo reminded those who saw it — and it is a reasonable assumption that, by November, most voters will have seen it — that Obama represents a kind of change with which they may not be as comfortable.

And if that is the case, as surely as summer follows spring, someone will label such tactics “racially divisive” and the you-know-what will hit the fan.

But if there are racism charges tossed around, then it is by no means clear whether they will help or hurt Obama’s candidacy. And it is not clear that we will even know at the time of a specific incident how it will play.

It is worth remembering that Willie Horton — the convicted murderer who killed again after he was given a furlough under a program enacted by Massachusetts Gov. Michael Dukakis — became a big issue in the 1988 presidential campaign. Democrats claimed the use of the case against Dukakis was unfair and the Republicans had a duty to avoid using a racially inflammatory example precisely because Horton was black.

If the election results are any indication, most voters apparently saw things differently. They saw it as a valid example of a program for which Dukakis was responsible that had not worked, and they did not agree that discussion of the case should be avoided because of Horton’s race.

Similarly, in the early 1990s, affirmative action became a major flash point during some campaigns as Republicans attacked government programs that they argued gave minorities an unfair advantage for jobs, contracts and college admission. Democrats also labeled such attacks out of bounds, but the issue seemed to work politically for the GOP, too.

The point is, labeling a political effort as racially divisive can be risky, depending on how the American people see it.

The Republican National Committee has reportedly begun studying how to criticize Obama without causing a backlash against McCain.

No doubt that information will come in handy in the coming months, because the picture of Obama in Somali dress is almost certain to be the first of many incidents that will determine the new rules of the road.

Peter A. Brown is the assistant director of the Quinnipiac University Polling Institute.